Compact anti-fuse memory cell using CMOS process

ABSTRACT

A compact CMOS anti-fuse memory cell. In one aspect, an apparatus includes an N-well and an anti-fuse cell formed on the N-well. The anti-fuse cell includes a lightly doped drain (LDD) region deposited in the N-well, an oxide layer deposited on the N-well and having an overlapping region that overlaps the LDD region, and a control gate deposited on the oxide layer, wherein a bit of the anti-fuse cell is programmed when a voltage difference between the control gate and the LDD region exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the LDD region, and wherein the leakage path is confined to occur in the overlapping region.

PRIORITY

This application is a continuation in part (CIP) of a U.S. patent application having application Ser. No. 15/096,170, filed on Apr. 11, 2016, and entitled “CMOS ANTI-FUSE CELL.”

This application claims the benefit of priority based upon U.S. Provisional Patent Application having Application No. 62/268,493, filed on Dec. 16, 2015, and entitled “COMPACT ANTI-FUSE MEMORY CELL COMPATIBLE TO LOGIC PROCESS” and U.S. Provisional Patent Application having Application No. 62/414,656, filed on Oct. 28, 2016 and entitled “COMPACT ANTI-FUSE MEMORY CELL COMPATIBLE TO LOGIC PROCESS”, both of which are hereby incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The exemplary embodiments of the present invention relate generally to the field of semiconductors and integrated circuits, and more specifically to memory and storage devices.

BACKGROUND OF THE INVENTION

Anti-fuse cell technology using a standard CMOS logic process is attractive due to its low manufacturing cost especially for embedded memory applications. The conventional anti-fuse cell using an NMOS device has low-cost, is easy-to-shrink, and provides high-density for advanced process nodes. Therefore, it is a good choice over other embedded memory devices, such as the flash memory. However, conventional anti-fuse cells do have several limitations. For example, because the anti-fuse cell can be programmed only one time, it is not suitable for systems that require re-programmability. To compensate for being only one-time programmable, conventional anti-fuse cells use multiple spare anti-fuse array blocks, which increase the overall die size and cost. Additionally, conventional anti-fuse cells may require isolation regions between cells due to the way the cells are programmed. This additional isolation may also increase the overall array size.

FIG. 1A shows a top view of a conventional N-channel anti-fuse cell structure 100 based on a standard CMOS logic process. The cell structure 100 includes a polysilicon select gate (SG) 101, a polysilicon control gate (CG) 102, N+ diffusions 103, a bit line contact 104, and a field isolation 105, such as a STI (Shallow-Trench-Isolation) oxide. A cross section indicator 111 is also shown.

FIG. 1B shows a cross section view of the cell structure 100. The cross section view is taken at cross section indicator 111 and further shows N+ diffusions 103 a, 103 b, and 103 c, P-well 110, and gate oxides 106 and 107. In this cross section view, a metal bit line (BL) 109 connected to the bit line contact 104 has been added. The CG 102 and the N+ diffusions (103 b and 103 c) form transistor 113, which may be a low voltage device, such as a 3V device. The transistor 112 is similarly formed.

During programming of the transistor 113, a 5V signal is applied to the CG 102, a 0V signal is applied to the bit line 109, and VDD (e.g., 3V) is applied to the SG 101. The 5V signal applied to the CG may be applied from 5V I/O devices that are normally available in a standard CMOS logic process. With these bias conditions, 0V passes from the BL 109 to the N+ diffusion 103 b. The 5V signal applied to the CG 102 will turn on the channel of transistor 113 and allow 0V to pass to the channel region 114. Because the voltage difference between the CG 102 and channel region 114 exceeds the breakdown voltage of the gate oxide 107, the gate oxide 107 will rupture (or breakdown) and cause the CG 102 to be shorted to the channel region 114 (this is referred to as ‘on-cell’). Thus, transistor 113 is programmed as an “on-cell.”

Meanwhile, the 3V signal is applied to cells associated with other (or unselected) bit lines (not shown). This 3V signal will be passed to the channel regions of these unselected cells. This prevents the voltage difference between the CGs and associated channels of these unselected cells from exceeding the gate oxide breakdown voltage, and thus the gate oxides of these unselected cells will not be ruptured (this is referred to as ‘off-cell’).

During a read operation, a positive voltage, such as 3V, is applied to the CG 102 and the BL 109 is biased at a lower voltage, such as 1V. If the cell (e.g., transistor 113) is an on-cell, current will flow from the CG 102 through the gate oxide breakdown region and to the BL 109. If the cell is an off-cell, there will be no current flowing.

When the channel 114 is turned on during programming of transistor 113, the oxide breakdown may occur at any location along the channel 114, such as breakdown path 108 a near the drain of transistor 113, breakdown path 108 b in the middle of the channel 114, or breakdown path 108 c near the source of transistor 113. The location of the breakdown path is dependent on where the oxide 107 is defective.

During a read operation, the channel 114 must be turned on to allow the leakage current to flow from CG 102 through the channel 114 to the drain diffusion 103 b. Since the breakdown may occur in the middle of the channel 114 (e.g., 108 b) or near the source side diffusion 103 c (e.g., 108 c), the source diffusion 103 c must be isolated from the adjacent cell by the field isolation 105. Otherwise, if the selected cell (e.g., transistor 113) is an off-cell but the adjacent cell is on-cell that has an oxide breakdown path near the source diffusion 103 c, current may flow from the adjacent cell to the selected cell and cause a read error. This condition may occur even if the adjacent cells' channel is turned off.

Moreover, during a program operation, because the selected cell's CG channel is turned on, the adjacent cell's CG voltage may leak to the selected cell and cause a programming failure. As a result, the cell's source region 103 c must be isolated to prevent these situations. However, the extra field isolation region (e.g., region 105) increases the cell size. It should also be noted that because the cell's channel is on during programming, the gate oxide breakdown may occur in different locations, and therefore different on-cell currents may result due to the different channel resistances that result. There may also be different cell characteristic resulting from different manufacturing processes and/or foundries.

It is therefore desirable to have an anti-fuse cell with uniform and less process dependent on-cell current and which avoids the use of additional isolation between cells to reduce size and cost.

SUMMARY

In various exemplary embodiments a novel anti-fuse cell structure is disclosed with uniform and less process dependent on-cell current and which avoids the use of additional isolation between cells to reduce size and cost. The cell can be manufactured using a standard CMOS logic process resulting in reduced cost for embedded memory applications.

In one aspect, an apparatus includes an N-well and an anti-fuse cell formed on the N-well. The anti-fuse cell includes a drain P+ diffusion deposited in the N-well, a source P+ diffusion deposited in the N-well, an oxide layer deposited on the N-well and having an overlapping region that overlaps the drain P+ diffusion. A control gate deposited on the oxide layer. A data bit of the anti-fuse cell is programmed when a voltage difference between the control gate and the drain P+ diffusion exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the drain P+ diffusion. The leakage path is confined to occur in the overlapping region.

In another aspect, a method is provided for operating an anti-fuse cell comprising a drain P+ diffusion deposited in an N-well, a source P+ diffusion deposited in the N-well, an oxide layer deposited on the N-well and having a first overlapping region that overlaps the drain P+ diffusion and a second overlapping region that overlaps the source P+ diffusion, and a control gate deposited on the oxide layer. The method comprises applying a first voltage to the control gate to turn off a channel region of the anti-fuse cell. The method also comprises applying a second voltage to the drain P+ diffusion wherein a first bit of the anti-fuse cell is programmed when a voltage difference between the first and second voltages exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the drain P+ diffusion. The leakage path is confined to occur in the first overlapping region.

In another aspect, an apparatus is provided that comprises an N-well and an anti-fuse cell formed on the N-well. The anti-fuse cell includes a lightly doped drain (LDD) region deposited in the N-well, an oxide layer deposited on the N-well and having an overlapping region that overlaps the LDD region, and a control gate deposited on the oxide layer. A bit of the anti-fuse cell is programmed when a voltage difference between the control gate and the LDD region exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the LDD region. The leakage path is confined to occur in the overlapping region.

In another aspect, an apparatus is provided that comprises an N-well, a select gate oxide coupled between a select gate and the N-well, a control gate oxide coupled between a control gate and the N-well, and an integrated spacer between the select gate and the control gate. The integrated spacer separates the select gate from the control gate by less than 20 nm. A bit of the anti-fuse cell is programmed when a voltage difference between the control gate and a channel region of the select gate exceeds a voltage threshold of the control gate oxide and forms a leakage path from the control gate through the control gate oxide to the channel region. The leakage path is confined to occur in a proximity region of the control gate oxide and the channel region.

Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments of the present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.

FIGS. 1A-B show a conventional anti-fuse cell structure;

FIGS. 2A-D show exemplary embodiments of a novel anti-fuse cell structure and associated bias conditions;

FIGS. 3A-B show exemplary embodiments of a novel anti-fuse cell structure that enables two bits to be programmed in one cell;

FIGS. 4A-B shows exemplary embodiments of a novel anti-fuse cell structure that utilizes additional cells to provide programmability;

FIGS. 5A-B shows exemplary embodiments of anti-fuse cell structures that provide programmability;

FIG. 6 shows an exemplary cell size comparison chart 600 that illustrates the cell size of conventional devices compared to the cell size of devices implemented with the novel anti-fuse cell structures shown in FIG. 5A-B;

FIGS. 7A-D show alternative exemplary embodiments of an anti-fuse cell structure and associated bias conditions;

FIGS. 8A-B show exemplary embodiments of novel anti-fuse cell structures;

FIGS. 9A-B show exemplary embodiments of novel anti-fuse cell structures;

FIG. 10 shows an exemplary embodiment of a method for programming a novel anti-fuse cell structure;

FIGS. 11A-B show exemplary embodiments of novel anti-fuse cell structures;

FIGS. 12A-F shows an exemplary process for forming the cell structure shown in FIG. 11A;

FIG. 13 shows an exemplary embodiment of a novel anti-fuse cell structure;

FIG. 14 shows an exemplary embodiment of a novel anti-fuse cell structure;

FIG. 15 shows an exemplary embodiment of a novel anti-fuse cell structure that combines the features of the cell structure embodiments shown in FIGS. 13-14;

FIG. 16 shows an exemplary embodiment of a novel anti-fuse cell structure;

FIG. 17 shows an exemplary embodiment of a novel anti-fuse cell structure; and

FIG. 18 shows an exemplary embodiment of a method for programming a novel anti-fuse cell structure;

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are described herein in the context of a process, device, method, and apparatus for providing an anti-fuse cell device.

Those of ordinary skilled in the art will realize that the following detailed description is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the exemplary embodiments of the present invention as illustrated in the accompanying drawings. The same reference indicators (or numbers) will be used throughout the drawings and the following detailed description to refer to the same or like parts.

FIGS. 2A-D show exemplary embodiments of a novel anti-fuse cell and associated bias conditions.

FIG. 2A shows a top view of an exemplary embodiment of a CMOS anti-fuse cell structure 200. The cell structure 200 includes a polysilicon SG 201, a polysilicon CG 202, P+ diffusions 203, a bit line contact 204, and a field isolation 205, such as STI (Shallow-Trench-Isolation) oxide. A cross section indicator 211 is also shown.

FIG. 2B shows a cross section view of the cell structure 200. The cross-section view is taken at cross section indicator 211 and further shows P+ diffusions 203 a, 203 b, and 203 c, gate oxides 206 and 207, and N-well 210. In this cross-section view, a metal BL 209 connected to the bit line contact 204 has been added. The CG 202 and the P+ diffusions (203 b and 203 c) form transistor 213, which may be a low voltage device, such as a 3V device. The transistor 212 is similarly formed.

During programming, a positive voltage is applied to the select CG 202 that is higher than the breakdown voltage of the gate oxide 207, such as 5V to 7V. For simplicity and clarity of the following description, a 5V signal will be used as example. The 5V may be derived from 5V I/O devices that are normally available in standard CMOS logic processes. In this way, no extra high voltage device is required, this can be referred to as ‘zero-cost added’ technology. A low voltage signal is applied to the BL 209, such as 0V for example, and the SG 201 is also supplied with 0V to pass 0V from the BL 209 to the P+ diffusion 203 b. The passed voltage may become approximately 1V due to the PMOS threshold voltage drop. In another embodiment, the SG 201 may be supplied with a negative voltage (such as −1V) to fully pass 0V to the P+ diffusion 203 b. However, this may require that negative charge pump circuitry be added.

Because CG 202 is supplied with 5V, it will turn off the channel 214 of transistor 213. The voltage difference between the CG 202 and drain P+ diffusion 203 b exceeds the breakdown voltage of the gate oxide 207. Since the channel 214 is off, the gate oxide 207 in the overlapping region 215 of the CG 202 and the drain P+ diffusion 203 b will breakdown and cause a short 208 between the CG 202 and the drain P+ diffusion 203 b. Thus, the cell is programmed and referred to as an ‘on-cell’. It should be noted that due to the channel region 214 being turned off, it will remain at the voltage of N-well 210, such as 3V, and thus the breakdown of the gate oxide 207 will not happen in the channel region 214 so that the breakdown is confined to the overlapping region 215 of the CG 202 and the drain P+ diffusion 203 b.

Meanwhile, unselected bit lines are supplied with 3V that will be passed to the drain P+ diffusions of the unselected cells. This reduces the voltage difference between the CG and the drain P+ diffusions of the unselected cells to be only 2V. Thus, the gate oxide of the unselected cells will not breakdown and the cells remains as ‘off-cells’.

In an exemplary embodiment, because the cell's channel is turned off during programming, the cell does not need field isolation on its source side. For example, the cell does not need the additional field isolation 105 shown in FIG. 1B. This reduces the cell size by approximately 20% when compared to conventional cells that use the additional field isolation.

For the unselected CGs, they can be either connected to 0V, 3V, 5V, or floating. When supplied with 0V, 3V, or floating, since the voltage does not exceed the gate oxide's breakdown voltage, it will not cause the oxide breakdown. When it is supplied with 5V, because the cell's channel is turned off, the channel remains at the N-well's voltage level such as 3V, thus the cell's gate oxide will not breakdown. This condition is advantageous because it allows all the CGs to be connected together. This eliminates the need for a CG decoder, which significantly saves silicon area especially since the CG decoder has to be made of 5V devices.

During a read operation, the CG 202 is supplied with a positive voltage, such as 3V, and the BL 209 is biased at a lower voltage, such as 1V. The SG 201 is supplied with 0V. If the cell (e.g., transistor 213) is an on-cell, a current that can be detected will flow from the CG 202 to the BL 209. If the cell is an off-cell, there will be no current flowing. Since this read condition turns off the channel region 214 of the cell (e.g., transistor 213), the cell will not be affected by the adjacent cell through the source P+ diffusion 203 c. It should also be noted that there are more than one set of bias conditions that may be supplied to the disclosed cell structure in accordance with the exemplary embodiments.

FIG. 2C shows an exemplary table of bias conditions that may be used to program the novel anti-fuse cell shown in FIG. 2B. The bias conditions assume the use of 3V devices. As illustrated in FIG. 2C, in Program 1 the SG 201 is supplied with 0V or −1V to select the anti-fuse cell 213, and 3V to deselect. When the cell 213 is selected, the CG 202 is supplied with 5V and the BL 209 is supplied with 0V to program the “on-cell” condition. In Program 2, the SG 201 is supplied with −2V to select the anti-fuse cell 213, and 1V to deselect. When the cell 213 is selected, the CG 202 is supplied with 3V and the BL 209 is supplied with −2V to program the “on-cell” condition. In Program 3, the cell's channel is turned on during programming and thus may be less preferred. For example, in Program 3 the SG 201 is supplied with 0V to select the anti-fuse cell 213, and 3V to deselect. When the cell 213 is selected, the CG 202 is supplied with −2V and the BL 209 is supplied with 3V to program the “on-cell” condition. In Program 4 a 5V junction breakdown is used, which may be implemented by using a 5V device's source and drain implant or (Lightly Doped Drain) LDD implant in a 3V device. For example, in Program 4 the SG 201 is supplied with 0V or −1V to select the anti-fuse cell 213, and 3V to deselect. When the cell 213 is selected, the CG 202 is supplied with 5V and the BL 209 is supplied with 0V to program the “on-cell” condition.

FIG. 2D shows an exemplary table of bias conditions that may be used to read the novel anti-fuse cell shown in FIG. 2B. The Read 1 bias conditions are preferred since the Read 2 bias conditions will turn on the cell's channel, which is less preferred. In Read 1, the SG 201 is supplied with 0V to select the anti-fuse cell 213, and 3V to deselect. When the cell 213 is selected, the CG 202 is supplied with 3V and the BL 209 is supplied with 1V to read the cell 213. For example, if the cell 213 is an “on-cell” current will flow from the CG 202 to the BL 209 when the Read 1 bias conditions are applied. In Read 2, the SG 201 is supplied with 0V to select the anti-fuse cell 213, and 3V to deselect. When the cell 213 is selected, the CG 202 is supplied with 0V and the BL 209 is supplied with 1V to read the cell 213. For example, if the cell 213 is an “on-cell” current will flow from the CG 202 to the BL 209 when the Read 2 bias conditions are applied.

It should be noted that the voltages shown in the tables are exemplary and that other suitable voltages may be used depending on the technologies and processes. For example, in another embodiment using 1.8V technology, the cell may be made of 1.8V devices, and the CG 202 may be supplied with 3V to breakdown the cell's gate oxide for programming. The CG voltage may be supplied by 3V I/O devices. Also notice, according to the invention, the CG 202 may have the same or a different channel length as the SG 201. Moreover, the devices may or may not have double-side or single-side LDD (Lightly Doped Drain), Halo implantation, or any other features that usually applied to the CMOS process. Moreover, although the CMOS logic process is preferred to realize zero-cost added embedded memory solution, it is obvious the exemplary embodiments of the invention can be applied to any other processes such as SOI, FinFET, BiCMOS, and many others. The above descriptions are applicable to all the embodiments in accordance with the invention. For simplicity, they will not be repeated in this description.

FIGS. 3A-B shows exemplary embodiments of a novel anti-fuse cell structure 300 that enables two bits to be programmed in one cell.

FIG. 3A shows a top view of the novel anti-fuse cell structure 300. In the exemplary embodiment shown in FIG. 3A, two cells 320 a and 320 b are shown. The cell structure 300 includes polysilicon SGs 301 a and 301 b, a polysilicon CG 302, P+ diffusions 303, bit line contacts 304 a and 304 b and field isolation 305, such as STI (Shallow-Trench-Isolation) oxide. The cells in FIG. 3A have a structure that is similar to the cell shown in FIG. 2A except the cells 320 a and 320 b share the CG 302. A cross section indicator 311 is also shown.

FIG. 3B shows a cross section view of the cell structure 300. The cross-section view is taken at cross section indicator 311 and further shows the P+ diffusions 303 a, 303 b, 303 c, and 303 d, gate oxides 306 a, 306 b, and 307, and N-well 310. In this cross-section view, a metal BL 309 connected to the bit line contacts 304 b and 304 b has been added. The CG 302 and the P+ diffusions (303 b and 303 c) form transistor 312, which may be a low voltage device, such as a 3V device. The transistors 313 and 314 are similarly formed.

To program the cell 320 a, the CG 302 is supplied with 5V, the SG 301 a is supplied with 0V, and BL 309 is supplied with 0V. This turns off the channel of transistor 312 and confines the gate oxide 307 to breakdown (as shown at 308 a) in the overlapping region of the CG 320 and drain P+ diffusion 303 b. Meanwhile, the unselected SG 301 b is supplied with 3V to turn off the transistor 314. The drain P+ diffusion 303 c remains floating at 3V and thus based on the small voltage difference between the CG 302 and drain P+ diffusion 303 c the gate oxide 307 will not breakdown due to this voltage differential.

Similarly, to program the cell 320 b, the SG 301 b is turned on to pass 0V from the BL 309 to the diffusion 303 c, which results in a voltage difference that causes the gate oxide 307 in the overlapping region of the CG 302 and drain P+ diffusion 303 c to breakdown as illustrated at 308 b. Since the channel of the transistor 312 is off, the oxide breakdown is confined to the overlapping region of the CG 302 and drain P+ diffusion 303 c. Meanwhile, the SG 301 a is turned off so the cell 320 a will not be affected. It should be noted that there are many sequences with which to apply the programming biases. For example, in one embodiment, the CG 302 and the BL 309 are supplied with 3V, and then the SG 301 a and SG 301 b are supplied with 0V to pass 3V to the drain diffusions 303 a and 303 b, etc. Then, the BL 309 and the SG 301 a are supplied with 0V, and then the selected CG (or all CG in another embodiment) is raised from 3V to 5V to effectuate programming.

It should be noted that the above-described method is just one example for programming two bits in the novel anti-fuse cell structure. It will be obvious to one with skill in the art that many different methods may be used with the various exemplary embodiments.

During a read operation, the CG 302 is supplied with a positive voltage, such as 3V for example. The selected SG is supplied with 0V and the unselected SG is supplied with 3V. If the selected cell is an on-cell, the current will flow from the CG 302 to the BL 309. If the cell is an off-cell, there will be no current flowing. It should be noted that during both program and read operations, because the CG 302 is turned off, the two cells can be programmed and read independently without affecting each other. Since two on-cell states can be programmed using only three devices, this embodiment reduces cell size by approximately 36% when compared with conventional anti-fuse devices.

FIGS. 4A-B shows exemplary embodiments of a novel anti-fuse cell structure 400 that utilizes additional cells to provide programmability.

FIG. 4A shows a top view of the novel anti-fuse cell structure 400. This embodiment allows the anti-fuse cell to be programmed more than one time. This function is important because typical anti-fuse cells can be programmed one time only. If a system needs re-programmability, conventional systems contain multiple ‘spare’ array blocks that result in higher costs and a large increase in array area. The embodiment shown in FIG. 4A does not use additional array blocks and therefore has lower cost and smaller array area when compared to conventional devices.

In the exemplary embodiment shown in FIG. 4A, the cell 400 includes a select gate 401 and multiple control gates 402 a and 402 b , P+ diffusions 403, bit line contact 404 and field isolation 405, such as STI (Shallow-Trench-Isolation) oxide. A cross section indicator 411 is also shown.

FIG. 4B shows a cross section view of the cell structure 400. The cross-section view is taken at cross section indicator 411 and further shows the P+ diffusions 403 a, 403 b, 403 c, and 403 d, gate oxides 408 a, 408 b, and 406, and N-well 410. In this cross-section view, a metal BL 409 connected to the bit line contact 404 has been added. The CG 402 a and the P+ diffusions (403 b and 403 c) form transistor 412, which may be a low voltage device, such as a 3V device. The transistors 413 and 414 are similarly formed.

In an exemplary embodiment, the cell 400 can be programmed multiple times without increasing the size of the array multiple times as in conventional devices. For example, assuming both cells 412 and 414 are not programmed, the cell 414 can be programed by suppling the BL 409 with 0V and suppling the SG 401 and CG 402 a with 0V. This will pass 0V to the drain P+ diffusion 403 c. The CG 402 b is supplied with 5V, which turns off its channel region. This will cause the gate oxide in 407 b to breakdown (as shown at 408 b) in the overlapping region between the CG 402 b and the P+ diffusion 403 c due to the voltage difference. Thus, the cell 414 is programmed to be an on-cell.

To read the cell (414), the SG 401 and CG 402 a are supplied with 0V and the CG 402 b is supplied with 3V. If the cell 414 is an on-cell, current will flow from the CG 402 b to the BL 409. If the cell (414) is not programmed, there will be no current flow to the BL 409.

In the event that the data needs to be updated, the new data can be programmed into the cell (412) and the cell (414) can be disabled. To program cell (412), the BL 409 and SG 401 are supplied with 0V and the CG 402 a is supplied with 5V. These conditions will turn off the channel associated with transistor 412 and cause the gate oxide 407 a to breakdown (as shown at 408 a) in the overlapping region between the CG 402 a and the P+ diffusion 403 b due to the voltage difference. To read the cell (412), the SG 401 is supplied with 0V and CG 402 a is supplied with 3V. Under these conditions, current will flow from CG 402 a to the BL 409 if it is an on-cell, otherwise no current will flow to the BL 409.

It should be noted that since the channel 415 of cell (412) is turned off during both program and read operations, the data of the cell (414) will not affect the cell (412). It should further be noted that in various exemplary embodiments, the programming sequence is conducted by programming the cell located furthest from the BL contact 404, which in this case is cell 414. Then, reprogramming can occur by programming the cell adjacent to the programmed cell moving toward the BL contact 404, which in this case in cell 412. Once the cell 412 is programmed, the cell 414 will not be able to be read correctly and therefore can be disabled. Thus, the cells are programmed in sequence from the furthest to the closest moving toward the BL contact 404. Although the exemplary cell structure shown in FIGS. 4A-B contains two cells (e.g., 412 and 414), it is obvious that the cell structure may be extended to contain any number of cells and exhibit the same novel features.

FIGS. 5A-B shows exemplary embodiments of anti-fuse cell structures that provide programmability.

FIG. 5A shows an exemplary embodiment of a cell structure that contains one SG 501 and multiple control gates 502 a-n that are part of multiple anti-fuse cells. BL contact 504 is also shown. Thus, the cell structure of FIG. 5A may be used to provide multiple anti-fuse cell programming by programming cells associated with the control gates 502 a-n. The programming order would be to start programming with cell 502 n, and then when reprogramming is desired, to program cell 502 n-1 and then cell 502 n-2, etc. Thus, the cells are programmed starting with the cell furthest from the BL contact 504 and programming adjacent cells moving toward the BL contact 504.

FIG. 5B shows another exemplary embodiment of a cell structure that contains one SG 501, BL contact 504 and multiple control gates 502 a-n. This cell structure is similar to that shown in FIG. 5A except the control gate 502 n is shared with an adjacent cell 503 (not shown in detail) However, the sharing of the control gate 502 n with an adjacent cell can be performed as described with reference to FIG. 3A-B.

It should be noted that in the embodiments shown in FIG. 5A-B, that although the cell structures according to the invention only require one SG, it is possible in other embodiments to utilize more than one SG. For example, in another embodiment, the device 502 n may be used as source side SG such that the devices 502 a to 502 n-1 represent the novel anti-fuse cells.

FIG. 6 shows an exemplary cell size comparison chart 600 that illustrates the cell size of the conventional devices versus the cell size of devices implemented with the novel anti-fuse cell structure shown in FIG. 5A-B. The chart 600 shows that cell size can be reduced 36%, 50%, 54%, and 57%, with the cell structures constructed in accordance with the disclosed embodiments and containing 1 to 4 cells, respectively.

FIGS. 7A-D show alternative exemplary embodiments of an anti-fuse cell and associated bias conditions. As illustrated in FIG. 7A the cell structure is similar to that shown in FIG. 2A except that the P+ diffusions 203 a, 203 b, and 203 c shown in FIG. 2A are changed to N+ diffusions 703 and the N-well 210 shown in FIG. 2B is changed to a P-well 710 as shown in FIG. 7B. As shown in FIG. 7B, the BL 709, BL contact 704, SG 701, CG 702, and oxides 706 and 707 correspond to those shown in FIG. 2B.

FIG. 7C shows a table that illustrates an exemplary embodiment of bias voltages used to program the anti-fuse cell shown in FIGS. 7A-B. It should be noted that unlike the conventional cell, the programming operations for the cell shown in FIGS. 7A-B turns off the cell's channel, and thus the gate oxide breakdown 708 is confined to occur in the overlapping region of the CG 702 and N+ diffusion 703 b. This allows the cell to avoid having field isolation on the source side, and thus reduces the cell size.

As the table in FIG. 7C illustrates, the Program 1 bias conditions use a 5V junction breakdown voltage (e.g., voltage difference between CG 702 and BL 709), thus may not suitable for 3V or 1.8V devices. The Program 2 only requires a 3V junction breakdown voltage. However, the Program 2 needs the CG 702 to be supplied with a negative voltage, which may be generated by negative charge pump circuitry added to the device.

FIG. 7D shows a table that illustrates an exemplary embodiment of bias voltages used to read the anti-fuse cell shown in FIGS. 7A-B. Similar to the anti-cell shown in FIGS. 2A-B, the CG 702 will turn off the cell's channel during this read condition. It should be noted that the bias conditions shown in FIGS. 7C-D turn off the NMOS cell's channel during program and read operations. Therefore, this implementation allows the cell structures shown above to also utilize NMOS devices. For simplicity, these embodiments are analogous to those described above and therefore will not be shown in the figures or described in greater detail.

In an exemplary embodiment, the bias conditions of FIGS. 7C-D can be applied to the conventional device shown in FIGS. 1A-B. The bias conditions cause the gate oxide breakdown to be confined to occur in the overlapping region between the CG 102 and the drain diffusion 103 b. Because the cell's channel is turned off, the gate oxide breakdown will not occur in the regions 108 b or 108 c. As a result, the cell current characteristics will become more uniform for different processes and foundries. For simplicity, these embodiments will not be shown in the figures or described herein, however, these applications shall remain within the scope of the invention.

FIGS. 8A-B show exemplary embodiments of novel anti-fuse cell structures. FIG. 8A shows an anti-fuse cell structure implemented with a bulk FinFET process. FIG. 8B shows an anti-fuse cell structure implemented with a SOI FinFET process. The embodiments shown in FIGS. 8A-B include select gates 801 a and 801 b and control gate 802. The structures also include silicon fins 803 a and 803 b that connect to a metal bit line (not shown) on top of the cells. An insulator 804, such as STI, and a substrate 805 are also shown. It should be noted that in an exemplary embodiment, the cells may be implemented using traditional junction transistors, and in another exemplary embodiment, the cells may be junction-less transistors.

FIG. 9A shows an exemplary embodiment of a novel anti-fuse cell structure 900. The cell structure 900 is similar to the cell structure shown in FIG. 2B, except that the P+ diffusion 203 c is replaced with STI region 205. The STI region 205 acts to isolate the channel 214 from the adjacent cell 902. The cell structure 900 can be programmed and read as in the previous embodiment shown in FIG. 2B. For example, in accordance with the invention, during programming the gate oxide of the cell 213 will breakdown in the overlapping region 208 of the CG 202 and the P+ diffusion 203 b. For ease of understanding, the reference numerals of FIG. 9A are the same as those in FIG. 2B and therefore the description with respect to FIG. 2B applies also to FIG. 9A. In an exemplary embodiment, the programming bias conditions shown in FIG. 2C can be used to program the cell structure 900.

FIG. 9B shows an exemplary embodiment of a novel anti-fuse cell structure 904. The cell structure 904 is similar to the cell structure shown in FIGS. 3B, except that an STI region 205 is included that divides the channel 214 of the control gate 202 into channels 214 a and 214 b. The cell structure 904 can be programmed and read as in the previous embodiment shown in FIG. 3B. For example, in accordance with the invention, during programming the gate oxide 207 of the cell 213 will breakdown (e.g., 208 a and 208 b) in the overlapping regions 215 a and 215 b of the CG 202 and the P+ diffusions 203 b and 203 c, respectively. In an exemplary embodiment, the programming bias conditions shown in FIG. 2C can be used to program the cell structure 904.

FIG. 10 shows an exemplary embodiment of a method 1000 for programming one or two bits of a novel anti-fuse cell constructed in accordance with the preset invention. For example, the method 1000 is suitable for programming the anti-fuse cell shown in FIG. 3B in accordance with the programming biasing conditions shown in FIG. 2C. For example, the anti-fuse cell comprises a drain P+ diffusion 303 b deposited in an N-well 310, a source P+ diffusion 303 c deposited in the N-well 310, an oxide layer 307 deposited on the N-well and having a first overlapping region that overlaps the drain P+ diffusion 303 b and a second overlapping region that overlaps the source P+ diffusion 303 c, and a control gate 302 deposited on the oxide layer 307.

In an exemplary embodiment, to program the anti-fuse cell, the Program 1 bias conditions shown in FIG. 2C can be used. For example, in the Program 1 bias conditions, the bit line is set to 0 volts, the control gate is set to 5 volts, the select gate associated with the bit to be programmed is set to 0 volts, and the N-well is set to 3 volts. Thus, the method 1000 includes the following operations.

At block 1002, a first voltage is applied to the control gate to turn off a channel region of the anti-fuse cell. For example, in an exemplary embodiment, a voltage level of 5 volts is applied to the control gate 302 to turn off its associated channel region. For example, the table shown in FIG. 2C illustrates Program 1 biasing conditions where a 5-volt level is applied to the control gate.

At block 1004, a second voltage is applied to the drain P+ diffusion wherein a first bit of the anti-fuse cell is programmed. For example, in an exemplary embodiment, the second voltage is passed from the bit line 309 through the select gate 301 a to the drain P+ diffusion 303 b. For example, in accordance with the Program 1 biasing conditions shown in FIG. 2C, a bit line voltage of 0-volts flows through the channel of the select gate 301 a and is applied to the drain P+ diffusion 303 b. It should be noted that the select gate channel is turned on due to the 0 volts applied to the select gate 301 a. The voltage difference between the first (5 volts) and second (0 volts) voltages exceeds a voltage threshold of the oxide layer 307 and forms a leakage path 308 a in the first overlapping region between the control gate 302 and the drain P+ diffusion 303 b. The leakage path is confined to occur in the first overlapping region.

At block 1006, a third voltage is applied to the source P+ diffusion wherein a second bit of the anti-fuse cell is programmed. For example, in an exemplary embodiment, the third voltage is passed from the bit line 309 through the select gate 301 b to the source P+ diffusion 303 d. For example, in accordance with the Program 1 biasing conditions shown in FIG. 2C, a bit line voltage of 0-volts flows to the source P+ diffusion 303 b when 0-volts is applied to the select gate 301 b. A voltage difference between the first (5-volts) and third (0-volts) voltages exceeds the voltage threshold of the oxide layer 307 and forms a second leakage path 308 b in the second overlapping region between the control gate 302 and the source P+ diffusion 303 c. The second leakage path 308 b is confined to occur in the second overlapping region between the control gate 302 and the source P+ diffusion 303 c.

Thus, the method 1000 operates to program one or two bits of a novel anti-fuse cell constructed in accordance with the preset invention. It should be noted that the method 1000 is exemplary and that the disclosed operations can be modified, deleted, added to, or otherwise rearranged within the scope of the embodiments. It should also be noted that the method 1000 utilizes Program 1 biasing conditions, however, the method may also utilize other biasing conditions as illustrated in FIG. 2C.

Additional Embodiments

FIG. 11A shows an exemplary embodiment of a novel cell structure 1100 in accordance with the present invention. The cell structure 1100 comprises two cells having polysilicon or metal gates 1101 a and 1101 b, where gate 1101 a is the select gate and gate 1101 b is the control gate. The cell structure 1100 also includes gate dielectric layers 1102 a and 1102 b , such as oxide. The gate oxide layers 1102 a and 1102 b may have the same thickness. In another embodiment, oxide layer 1102 b may have thinner gate oxide than gate oxide 1102 a. The cell 1100 also includes spacers 1103 a and 1103 c, and integrated spacer 1103 b. Below the spacers are LDD regions 1104 a-1104 c.

As illustrated in FIGS. 11A, the transistors' gates 1101 a and 1101 b of the cell structure 1100 are placed closer together than the typically specified design rule spacing. For example, consider a cell structure that does not have an integrated spacer (like 1103 b). In this case, the width of each spacer (e.g., one spacer coupled to the right side of the select gate 1101 a and one spacer coupled to the left side of the control gate 1101 b) can be designated (X). The space between the two spacers can be designated as (Y). Thus, the typical spacing between the select gate 1101 a and the control gate 1101 b will be (2X+Y). However, using the integrated spacer 1103 b with no space in the middle, the spacing between the select gate 1101 a and the control gate 1101 b will be smaller than 2×. For example, using a 28 nanometer (nm) process, X and Y can be approximately 20 nm and 40 nm, respectively. The actual X and Y values may vary dependent upon the foundries and tools used. Therefore, during the spacer-etching process, the spacers between the gates may not be completely separated. Instead, the integrated spacer 1103 b is formed. During the source and drain diffusions' ion implantation, the spacer 1103 b will block the diffusion region between the two gates. As a result, the diffusion between the two gates only has a LDD implant layer 1104 b but no source or drain implant. This may effectively increases the junction breakdown voltage because of the lighter concentration of the LDD implant layer 1104 b. Moreover, because the two gates 1101 a and 1101 b do not need to reserve spacing for two separated spacers and diffusion implantation, the space between the two gates 1101 a and 1101 b is reduced.

The cell structure 1100 also includes oxide or nitride spacers 1103 a and 1103 c and LDD (lightly doped drain) regions 1104 a-1104 c. The cell structure 1100 also includes source and drain diffusions 1105 a-1105 b, a substrate or well 1106, a bit line contact 1107, and a metal bit line 1108.

In various exemplary embodiments, the cell may be implemented using PMOS or NMOS transistors. In a PMOS embodiment, the diffusions 1105 a-1105 b and the LDD regions 1104 a-1104 c may have P-type doping and the region 1106 may be an N-well located in P-substrate. In an NMOS embodiment, the diffusions 1105 a-1105 b and LDD regions 1104 a-1104 c may have N-type doping and the region 1106 may be P-substrate.

In an exemplary embodiment, the program biasing conditions shown in FIG. 2C can be used to program the cell. For example, using Program 1 bias conditions, the control gate 1101 b is supplied with 5 volts, the bit line 1108 is supplied with 0 volts, and the select gate 1101 a is supplied with 0 volts, which operates to turn on the channel of the select gate 1101 a and pass the bit line voltage (e.g., 0 volts) to the LDD region 1104 b. With these bias conditions, the voltage difference between the control gate 1101 b and the LLD region 1104 b exceeds the threshold of the gate oxide layer 1102 b. This causes a breakdown 1111 that is confined to occur the overlapping region 1110 between the oxide 1102 b and the LDD region 1104 b. It should also be noted that other program biasing conditions as shown in FIG. 2C can be utilized to program the cell.

FIG. 11B shows an exemplary embodiment of a cell structure 1120 in accordance with the presents invention. The cell structure 1120 shown in FIG. 11B is similar to the cell structure 1100 shown in FIG. 11A except that a STI (shallow-trench-isolation) 1109 is formed in the source side of the cell in place of the LLD region 1104 c and the diffusion 1105 b.

FIGS. 12A-F shows an exemplary process for forming the cell structure 1100 shown in FIG. 11A.

FIG. 12A shows an exemplary embodiment of a first process step (or operation) wherein a substrate 1201, a gate oxide layer 1202 and a polysilicon layer 1203 are deposited in stacked layers.

FIG. 12B shows an exemplary embodiment of a second process step wherein transistor gates 1204 a and 1204 b are pattern-etched. In an exemplary embodiment, the gates 1204 a and 1204 b can have a shorter spacing than specified by design rules. For example, as described above, in an exemplary embodiment the spacing between the gates 1204 a and 1204 b will be smaller than 2×. For example, using a 28 nanometer (nm) process, X can be approximately 20 nm. However, in another exemplary embodiment describe below, the spacing 2× is smaller than 20 nm.

FIG. 12C shows an exemplary embodiment of a third process step wherein a LDD implantation is applied to form LDD regions 1206 a-1206 c.

FIG. 12D, shows an exemplary embodiment of a fourth process step wherein a spacer material 1207 is deposited over the gates 1204 a and 1204 b.

FIG. 12E shows an exemplary embodiment of a fifth process step wherein a spacer-etching is applied to form spacers 1208 a-1208 c. It should be noted that the spacer 1208 b between the gates 1204 a and 1204 b is integrated to form one space in this region.

FIG. 12F shows an exemplary embodiment of a sixth process step wherein source and drain implantations are applied to form diffusions 1209 a and 1209 b. In an exemplary embodiment, the above process steps are implemented by using standard logic processes without adding extra mask or process steps.

FIG. 13 shows an exemplary embodiment of a cell structure 1300 in accordance with the present invention. The cell structure 1300 includes polysilicon or metal gates 1301 a-1301 c, where the gates 1301 a and 1301 c are select gates and gate 1301 b is a control gate 1301 b. The cell structure 1300 also includes gate dielectric layers 1302 a-1302 c, which, for example, in can be an oxide. The cell structure 1300 also includes oxide or nitride spacers 1303 a-1303 d, LDD (lightly doped drain) regions 1304 a-1304 d, and source and drain diffusions 1305 a-1305 b. The cell structure 1300 also includes a substrate (or well) 1306, bit line contacts 1307 a and 1307 b, and a metal bit line 1308.

In this embodiment, the control gate 1301 b is shared by two select gates 1301 a and 1301 c. Thus, one or two bits can be programmed. For example, during programming using Program 1 bias conditions, the control gate 1301 b is supplied with a voltage to turn off its channel, thus the bit line 1308 voltage can be flow through the channels of the select gates 1301 a and 1301 c to LDD regions 1304 b and 1304 c independently. This will cause the gate oxide 1302 b to breakdown (e.g., 1310 and 1312) in the overlapped regions (e.g., 1311 and 1313) of the oxide 1302 b and LDD regions 1304 b and 1304 c. As a result, the control gate 1301 b can store two data bits in the two sides of the gate oxide 1302 b. During a read operation, the control gate 1301 b is supplied with a voltage to turn off its channel, thus the two sides' data can be read independently. Thus, FIG. 13 illustrates the special features of the integrated spacers 1303 b and 1303 c, and corresponding LDD regions 1304 b and 1304 c according to the invention.

FIG. 14 shows an exemplary embodiment of the cell structure 1400 in accordance with the present invention. The cell structure 1400 includes polysilicon or metal gates 1401 a-1401 d, where the gate 1401 a is a select gate and gates 1401 b-1401 d are control gates. The cell structure 1400 also includes gate dielectric layers 1402 a-1402 d, which, for example, in can be an oxide. The cell structure 1400 also includes oxide or nitride spacers 1403 a-1403 e, LDD (lightly doped drain) regions 1404 a-1404 e, and source and drain diffusions 1405 a-1405 b. The cell structure 1400 also includes a substrate (or well) 1406, bit line contact 1407, and a metal bit line 1408.

In this embodiment, the cell structure 1400 includes the select gate 1401 a and multiple control gates 1401 b-1401 d to allow multiple-time programming. For first time programming, the data is programmed to the first control gate 1401 d by breakdown 1414 of the gate oxide layer 1402 d in the overlapping region 1415. The reserved second and third control gates 1401 b and 1401 c are turned on as pass gates. For second time programming, the new data is programmed to the second control gate 1401 c by breakdown 1412 of the gate oxide layer 1402 c in the overlapping region 1413. The old data stored in the first control gate 1401 d is abandoned. For third time programming, the new data is programmed to the third control gate 1401 b by breakdown 1410 of the gate oxide layer 1402 b in the overlapping region 1411. The old data stored in the second control gate 1401 c is abandoned.

In this embodiment, during program and read operation, the selected control gate is supplied with a voltage to turn off its channel, thus the oxide breakdown will occur in the overlapping drain region of the control gate and the operations will not be affected by the abandoned control gates. It should also be noted that although the cell structure 1400 shown in FIG. 14 includes three control gates, in other embodiments, the cell structure may have any number of control gates. It should also be noted that the programming can be performed according to the program bias conditions shown in FIG. 2C.

FIG. 15 shows an exemplary embodiment of a novel cell structure 1500 that combines the features of the cell structure embodiments shown in FIGS. 13-14. The cell structure 1500 includes select gates 1501 a and 1501 e. A first control gate 1501 c stores two data in two sides of its gate oxide layer, and its operation is shared by the select gates 1501 a and 1501 e. For example, the control gate 1501 c is programmed by breakdowns 1514 and 1516 in the overlapping regions 1515 and 1517 similar to the cell structure shown in FIG. 13. A second control gate 1501 b and third control gate 1501 d are operated by the select gates 1501 a and 1501 e, respectively. For example, the control gate 1501 b is programmed by breakdown 1512 in the overlapping region 1513, and the control gate 1501 d is programmed by breakdown 1518 in the overlapping region 1519. These control gates are programmed similar to the cell structure shown in FIG. 14.

FIG. 16 shows an exemplary embodiment of a cell structure 1600 in accordance with the invention. The cell structure 1600 is similar to the cell structure shown in FIG. 11A, except that the LDD region 1104 b region is eliminated. When the spacing between the gates 1101 a and 1101 b is close enough, the inverted channels under the gates may be connected together, and therefore the LDD region 1104 b is not needed. For example, in an exemplary embodiment, the integrated spacer 1103 b has a width 1113 of approximately 20 nm or less. Thus, in an exemplary embodiment, during the process operation illustrated in FIG. 12B, the select gate 1204 a and the control gate 1204 b are spaced apart by 20 nm or less, which eliminates the LDD region 1206 b. During programming, for example using Program 1 bias conditions shown in FIG. 2C, the control gate 1101 b is supplied with 5 volts and the channel 1112 of the select gate 1101 a is turned on to allow the bit line voltage (e.g., 0 volts) to flow through the turned-on channel and appear in a proximity region 1115, which is close enough to the oxide 1102 b to cause a breakdown 1114 of the oxide 1102 b in the proximity region 1115. Thus, even without the LDD region, it is still possible to program the control gate 1101 b.

FIG. 17 shows an exemplary embodiment of a cell structure 1700 in accordance with the invention. The cell structure 1700 is similar to the cell structure shown in FIG. 13, (and can be forming using a process similar to that described with respect to FIG. 12) except that the LDD regions 1304 b and 1304 c are eliminated and the integrated spacers 1303 b and 1303 c are formed to have a width of 20 nm or less. During programming, for example using Program 1 bias conditions shown in FIG. 2C, the control gate 1301 b is supplied with 5 volts and the channel 1314 of the select gate 1301 a is turned on to allow the bit line voltage (e.g., 0 volts) to flow through the channel 1314 to the proximity region 1316 to cause a breakdown 1315 of the oxide 1302 b in the proximity region 1316. A second bit can be programmed when the control gate 1301 b is supplied with 5 volts and the channel 1317 of the select gate 1301 c is turned on to allow the bit line voltage (e.g., 0 volts) to flow through the channel 1317 to the proximity region 1319 to cause a breakdown 1318 of the oxide 1302 b in the proximity region 1319. It should be noted that the narrow “integrated spacer” can be used to eliminate the LDD regions in other exemplary embodiments of the invention.

FIG. 18 shows an exemplary embodiment of a method 1800 for programming one or two bits of a novel anti-fuse cell constructed in accordance with the preset invention. For example, the method 1800 is suitable for programming the anti-fuse cells shown in FIG. 13 and FIG. 17 in accordance with the programming biasing conditions shown in FIG. 2C. For example, the anti-fuse cell 1300 shown in FIG. 13 comprises control gate 1301 b and select gates 1301 a and 1301 c separated by integrated spacers 1303 b and 1303 c. The cell 1300 also includes oxide layers 1302 a-1302 c and LDD regions 1304 a-1304 d. For example, the anti-fuse cell 1700 shown in FIG. 17 comprises control gate 1301 b and select gates 1301 a and 1301 c separated by very narrow integrated spacers 1303 b and 1303 c, which are 20 nm or less wide. The cell 1700 also includes oxide layers 1302 a-1302 c. However, since the spacers are so narrow there are no LDD regions between the select and control gates. Instead, the channels of the select gates extend to the oxide layer 1302 b.

In an exemplary embodiment, to program the anti-fuse cells 1300 and 1700, the Program 1 bias conditions shown in FIG. 2C can be used. It should be noted that different programming bias (and read bias) conditions as illustrated in FIG. 2C can be used. For example, in the Program 1 bias conditions, the bit line is set to 0 volts, the control gate is set to 5 volts, the select gate associated with the bit to be programmed is set to 0 volts, and the N-well is set to 3 volts. Thus, the method 1800 includes the following operations.

At block 1802, a first voltage is applied to the control gate to turn off a channel region of the anti-fuse cell. For example, in an exemplary embodiment, a voltage level of 5 volts is applied to the control gate 1301 b to turn off its associated channel region. For example, the table shown in FIG. 2C illustrates Program 1 biasing conditions where a 5-volt level is applied to the control gate.

At block 1804, a second voltage is applied to either a drain LDD region or select gate channel to program a first bit of the anti-fuse cell. For example, in an exemplary embodiment, the second voltage is passed from the bit line 1308 through the select gate 1301 a to the drain LDD 1304 b or to the turned-on channel 1314 of the select gate 1301 a. For example, in accordance with the Program 1 biasing conditions shown in FIG. 2C, a bit line voltage of 0-volts flows through the channel of the turned-on select gate 1301 a and is applied to the drain LDD region 1304 b or is simply applied to the channel 1314. It should be noted that the select gate channel is turned on due to the 0 volts applied to the select gate 1301 a. The voltage difference between the first (5 volts) and second (0 volts) voltages exceeds a voltage threshold (e.g.,4 volts) of the oxide layer 1302 b and forms a leakage path (either 1310 or 1315) in a first overlapping region 1311 or proximity region 1316 between the oxide 1302 b and the drain LDD region 1304 b or between the oxide 1302 b and the channel 1314. Thus, the leakage path is confined to occur in the first overlapping 1311 or proximity region 1316.

At block 1806, a third voltage is applied to a source LDD region or a second select gate channel to program a second bit of the anti-fuse cell. For example, in an exemplary embodiment, the third voltage is passed from the bit line 1308 through the select gate 1301 c to the source LDD region 1304 c or to the turned-on channel 1317 of the select gate 1301 c (e.g., second select gate). For example, in accordance with the Program 1 biasing conditions shown in FIG. 2C, a bit line voltage of 0-volts flows through the channel of the turned-on select gate 1301 c and is applied to the source LDD region 1304 c or is simply applied to the channel 1317. It should be noted that the select gate channel is turned on due to the 0 volts applied to the select gate 1301 c. The voltage difference between the first (5 volts) and second (0 volts) voltages exceeds a voltage threshold of the oxide layer 1302 b and forms a leakage path (either 1312 or 1318) in a second overlapping region 1313 or proximity region 1319 between the oxide 1302 b and the source LDD 1304 c or between the oxide 1302 b and the channel 1317. Thus, the leakage path is confined to occur in the second overlapping region 1313 or proximity region 1319.

Thus, the method 1800 operates to program one or two bits of a novel anti-fuse cell constructed in accordance with the preset invention. It should be noted that the method 1800 is exemplary and that the disclosed operations can be modified, deleted, added to, or otherwise rearranged within the scope of the embodiments. It should also be noted that the method 1800 utilizes Program 1 biasing conditions, however, the method may also utilize other biasing conditions as illustrated in FIG. 2C.

While exemplary embodiments of the present invention have been shown and described, it will be obvious to those with ordinary skills in the art that based upon the teachings herein, changes and modifications may be made without departing from the exemplary embodiments and their broader aspects. Therefore, the appended claims are intended to encompass within their scope all such changes and modifications as are within the true spirit and scope of the exemplary embodiments of the present invention. 

What is claimed is:
 1. An apparatus, comprising: an N-well; and an anti-fuse cell formed on the N-well, wherein the anti-fuse cell includes a lightly doped drain (LDD) P-region deposited in the N-well, an oxide layer deposited on the N-well and having an overlapping region that overlaps the LDD P-region, and a control gate deposited on the oxide layer, wherein a bit of the anti-fuse cell is programmed when a positive voltage difference from the control gate to the LDD P-region exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the LDD P-region, and wherein the leakage path is confined to occur in the overlapping region.
 2. The apparatus of claim 1, wherein the oxide layer comprises a Shallow-Trench-Isolation (STI) oxide.
 3. The apparatus of claim 1, wherein the anti-fuse cell forms a low voltage device.
 4. The apparatus of claim 1, wherein the anti-fuse cell forms a PMOS device.
 5. The apparatus of claim 1, wherein the voltage threshold of the oxide layer is approximately 4 volts.
 6. The apparatus of claim 1, wherein during the programming of the bit, a control gate voltage on the control gate turns off a channel region associated with the control gate.
 7. The apparatus of claim 1, wherein during a read operation a second voltage difference between the control gate and the LDD causes a current to flow through the leakage path if the anti-fuse cell is programmed to be an on-cell, and causes no current to flow if the anti-fuse cell is programmed to be an off-cell.
 8. The apparatus of claim 1, further comprising a source LDD P-region deposited in the N-well and having an end region that overlaps the oxide layer at a second overlapping region.
 9. The apparatus of claim 8, wherein the source LDD P-region is coupled to a bit line.
 10. The apparatus of claim 9, wherein a second bit of the anti-fuse cell is programmed when a positive voltage difference from the control gate to the source LDD P-region exceeds a voltage threshold of the oxide layer and forms a second leakage path from the control gate to the source LDD P-region, and wherein the second leakage path is confined to occur in the second overlapping region.
 11. The apparatus of claim 9, further comprising one or more additional anti-fuse cells formed on the N-well, wherein the additional anti-fuse cells are coupled in series and share LDD regions for adjacent source and drain sides, wherein programming of the one or more additional anti-fuse cells starts with programming a selected anti-fuse cell, and wherein re-programming is performed by disabling a programmed anti-fuse cell and programming an adjacent anti-fuse cell.
 12. A method of operating an anti-fuse cell comprising a LDD P-region deposited in an N-well, an oxide layer deposited on the N-well and having a first overlapping region that overlaps the LDD P-region, and a control gate deposited on the oxide layer, the method comprising: applying a first voltage to the control gate to turn off a channel region of the control gate; and applying a second voltage to the LDD P-region, wherein a first bit of the anti-fuse cell is programmed when a positive voltage difference from the first voltage to second voltage exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the LDD region, and wherein the leakage path is confined to occur in the first overlapping region.
 13. The method of claim 12, wherein the anti-fuse cell further comprises a second LDD P-region deposited in an N-well and that overlaps the oxide layer at a second overlapping region, the method comprising: applying a third voltage to the second LDD P-region wherein a second bit of the anti-fuse cell is programmed when a positive voltage difference from the first voltage to the third voltage exceeds the voltage threshold of the oxide layer and forms a second leakage path from the control gate to the second LDD P-region, and wherein the second leakage path is confined to occur in the second overlapping region.
 14. An apparatus, comprising: an P-well; and an anti-fuse cell formed on the P-well, wherein the anti-fuse cell includes a lightly doped drain (LDD) N-region deposited in the P-well, an oxide layer deposited on the P-well and having an overlapping region that overlaps the LDD N-region, and a control gate deposited on the oxide layer, wherein a bit of the anti-fuse cell is programmed when a negative voltage difference from control gate to the LDD N-region exceeds a voltage threshold of the oxide layer and forms a leakage path from the control gate to the LDD N-region, and wherein the leakage path is confined to occur in the overlapping region.
 15. The apparatus of claim 14, wherein the anti-fuse cell forms an NMOS device. 